what wire go's to the iac
You can get this kind of answer very quickly through Google. Then you won't need to wait for one of us to have time to check the forum and respond.
I found this with the google search "eec-iv pinout mustang"
In that diagram, you can see the "Idle Speed Control" component. You can see that the power wire is red and is shared with most other sensors and components, like the fuel injectors. You can see then that the controlling wire is the ground wire, that it is White/Light Blue, and that it goes to eec-iv pin 21.
I found this the google search "Megasquirt 2 pinout".
Even though this is not mustang specific, you can see that it has a PWM idle control output, wired up the same way the factory has the mustang IAC wired up. With power shared with the other components, and a single ground control wire, which is going to pin 30.
Knowing for sure that the factory harness will be on pin21, you can trace that in your adapter and see that it would be going to megasquirt pin 30, which would jive with the MS2 diagram from the google search.
Not all trying to bust your balls, just trying to help show how to find this kind of information so that you don't have to wait so long for information.
By the way, that car must be a blast putting down that kind of power to the wheels. I think it's about time you made a video!